1. Field of the Invention
This invention relates to the cutting of glass sheets and more particularly to the cutting of glass sheets from a heated glass ribbon while maintaining quality optics in the cut glass sheets.
2a. Technical Considerations
The fabrication of flat or bent glass products conventionally requires two separate and distinct processes: primary processing and secondary processing.
The primary processing includes forming a glass ribbon from molten glass by either floating the molten glass on a bath of tin or pulling a sheet of glass from a molten glass bath. In the float forming process, the molten glass is deposited and floated on a molten tin bath to form a ribbon, for example as disclosed in U.S. Pat. No. 3,843,346 to Edge et al. The float glass ribbon starts to cool and its thickness is established while on the tin bath. The ribbon is thereafter lifted off of the tin bath and conveyed into an annealing zone where it is controllably cooled to a temperature below its strain point. In the sheet forming process, the thickness of the glass sheet is established while it is being pulled from the pool of molten glass and is cooled thereafter, for example as disclosed in U.S. Pat. No. 1,339,229 to Slingluff. After annealing, the ribbon is cut into individual glass sheets for subsequent secondary processing.
Secondary processing may include any number of additional glass sheet processing procedures. For example, the large sheets may be cut into smaller, more easily handleable sheets prior to cutting the glass sheet to its final shape. The final shape may be rectangular as in desk tops, glazing units, etc. or maybe more complex as in special glazing designs or automotive windows. After cutting the glass to shape it may be drilled and/or edged and/or bent to shape. The drilling and edging operations are generally performed while the glass is at room temperature. These operations may leave small chips in the glass edge that may act as stress concentration points and lead to subsequent failure and breakage. In the shaping operation, the cut glass is reheated to its heat deformable temperature, approximately 1150.degree. F. to 1250.degree. F. (621.degree. C. to 677.degree. C.) for typical soda-lime-silica glass. The heating and bending operations may be performed in any of a number of different methods. For example, glass sheets may be conveyed on rolls through a heating furnace for entry into a so called horizontal press bending station as in U.S. Pat. No. 4,197,108 to Frank et al. or conveyed on a pressurized gas bed, or gas hearth, through a heating furnace for entry into a bending station where the glass sheet is shaped by a drop forming process as depicted in U.S. Pat. No. 4,204,853 to Seymour or the glass sheet to conveyed curved forming rolls to be shaped by a roll forming process as taught in U.S. Pat. No. 4,139,359 to Johnson et al.
After the cold processing and/or heating for bending, the glass may be heat strengthened. In the case of room temperature glass, the glass sheet is reheated to a temperature above its annealing range and then rapidly cooled through the annealing range to its strain point to impart a temper in the glass sheet. In the case of bent glass, after the bending operation, the glass sheet is rapidly cooled to impart temper.
The present practice of fabricating glass products with acceptable optical quality such as windows, mirrors, and the like requires the practice of the primary and secondary processes. It is apparent that there would be significant cost savings if the primary or secondary process could be modified or the two processes could be integrated into a single process so as to eliminate certain steps, e.g. reheating, and improve the finished product, e.g. high quality glass edge. There would be additional cost savings if the primary processing included float forming of a glass ribbon so that the glass would have its final glass thickness and optical quality as it is removed from the tin bath without any additional stretching, grinding, or polishing operation.
2b. Patents of Interest
U.S. Pat. Nos. 3,189,424 to Brichard et al.; 3,486,673 to Madge; 3,690,527 to Bustrann et al.; 3,754,884 to McDavid et al.; and 3,875,766 to French teach the severing of a drawn glass ribbon. In each the glass ribbon is cooled so that a major surface of the glass is susceptible to scoring by a scribing apparatus and subsequently fractured along the scored line.
U.S. Pat. Nos. 1,550,428 to Corl; 1,560,077 to Gelstharp; and 2,243,149 to Despret teach the severing of a glass ribbon in a sheet glass operation while the ribbon is still in a plastic state. In particular, Corl teaches the use of severing ribs along each major surface of the ribbon to effect a "biting into" of the sheet disposed therebetween. In Gelstharp, a cutting wheel mounted on a carriage and movable along the transverse direction of the ribbon is used to sever the still plastic glass sheet ribbon. In Despret, a roller with a knife blade is rotated at a predetermined speed so that the blade cuts the soft glass.
U.S. Pat. No. 3,124,444 to Ritter, Jr. et al. teaches a method for continuously removing the marginal edge or border portions of a moving ribbon of glass while it is in a plastic condition. The pair of circular cutting discs are arranged at the edge of the ribbon so that the ribbon passes therebetween and the edge of the ribbon is cut from the remainder of the ribbon.
French Patent Publication No. 2 567 872 to Diaz et al. teaches a method for cutting glass sheets still in the plastic state. The glass passes over a pair of rotating knives which pinch the glass almost through its entire thickness. A first set of knives are positioned radially around a cutting roll to continuously cut the glass sheet in its longitudinal direction, and a second set of knives are positioned longitudinally along the length of the cutting rolls to periodically cut the glass sheet in the transverse direction.
U.S. Pat. Nos. 3,453,097 to Hafner; 3,543,979 to Grove et al.; 3,885,943 and 3,930,825 to Chui; and 3,935,419 to Lambert et al. teach the cutting of glass with a laser beam. In Hafner the parameters of the laser are selected so that the glass sheet absorbs the laser energy and converts it into sufficient heat to enable separation of the glass sheet into pieces along a line swept by the laser beam. In Grove, which teaches cutting of glass sheets from a continuous glass ribbon, a laser beam scans the glass to induce a stress field in the glass which causes a controlled fracture along the scanned line. In the Chui patents, lasers are used to vaporize the glass along the cutting line. In U.S. Pat. No. 3,930,825 a pair of lasers are used to cut shaped glass articles directly from a float glass ribbon. It is believed that the power of the laser and the time required by such a laser to vaporize the glass may be prohibitive in a high volume commercial operation.
U.S. Pat. No. 3,934,995 to French teaches a method for cutting discrete sheets from a glass ribbon as it emerges from a float line by subjecting a portion of the ribbon to a controlled quench to cool the glass, scribing the glass along the cooled line, and rapidly breaking the glass along the scribed line.
U.S. Pat. No. 3,193,367 to Giffen teaches a glass forming operation wherein a sheet of molten glass drawn from a pool of molten glass is positioned over a forming die, pressed into the die and severed by shearing elements that separate the glass from the remaining glass sheet during the pressing operation.
U.S. Pat. No. 3,584,773 to Grove teaches a method of cutting glass by employing high frequency dielectric heating of the glass through its thickness to cause a controlled fracture to run along the cutting line.
U.S. Pat. No. 3,687,956 to Oelke teaches a method and apparatus for cutting cooled glass plates by applying heat along a desired line of cut from a source which is spaced from the surface of the plate. The heat from a radiant heat source is restricted to narrow the path corresponding to the line of cut. By concentrating the heat, a large temperature differential is set up between the unheated portions of the glass sheet and the very limited heated area and the coincident stress buildup cases a break through the sheet.
U.S. Pat. No. 4,162,907 to Anderson teaches a method of cutting molten glass on a molten tin bath. An extruder applies the molten glass to the molten tin bath where a cutting frame is lowered over the bath to firmly hold the glass and a cutting mechanism cuts the molten glass into desired sizes. The molten glass is cut with a compressed air driven rotary blade which penetrates completely through the glass and into the molten tin.
U.S. Pat. No. 4,361,429 to Anderson et al. teaches a method and apparatus for forming articles from molten sheet glass. Glass is drawn from a pool of molten glass by a pair of rollers. The molten glass sheet is positioned over and conformed to a mold cavity by a combination of vacuum and/or gravity. While still in a molten condition, the sheet is pressed and cut into a finished article. A vacuum take-out lifts the finished article out of the mold.
These patents disclose glass cutting techniques, but they are not concerned with cutting a high optical quality glass ribbon that has been sufficiently cooled to allow handling of the glass without reducing its optical quality, but still hot enough so that the glass can be heat treated, i.e. tempered or annealed, without reheating the glass.